The present invention relates generally to an exposure apparatus, and more particularly to a light source apparatus used in an exposure apparatus for exposing an plate, such as a single crystal substrate of a semiconductor wafer etc. and a glass plate for a liquid crystal display (“LCD”). The present invention is suitable, for example, for an exposure apparatus that uses an extreme ultraviolet (“EUV”) light as a light source for exposure.
A reduction projection optical system using an EUV light as an exposure light (referred to as an “EUV exposure apparatus” hereinafter) has been developed to manufacture a fine semiconductor device that has very fine circuit pattern of 0.1 μm or less. The EUV light is a light with a wavelength of 10 nm to 15 nm shorter than that of an ultraviolet light.
The EUV exposure apparatus typically uses a laser plasma light source and a discharge plasma light source as the light source. See, for example, Japanese Patent Applications, Publication Nos. 2002-174700 and 2003-077698. The laser plasma light source irradiates a laser beam to a target material to generate a plasma and generates the EUV light. The discharge plasma light source generates a plasma by introducing gas to an electrode for discharging and generates the EUV light. The EUV light from the plasma is condensed at a condensing point, diverges from the condensing point, and enters a subsequent illumination optical system via an opening formed between the light source and the illumination optical system.
However, a material necessary for an emission, i.e., the target material or discharge gas, is supplied to a light source chamber that accommodates the plasma (an emission part). Therefore, an impurity included in those materials flows into the light source chamber. A hydrocarbon included in the impurity adheres a carbon as a contaminant to a part irradiated by the EUV light, and reduces a reflectance of an optical element, such as a condensing mirror.
Moreover, it is preferably to provide a window material that functions as a vacuum partition at the opening (connecting part) between the light source and the illumination optical system. However, a material that penetrates the EUV light or a material that can fully resist a heat load by the EUV light of power necessary for exposure does not exist. Therefore, the hydrocarbon included in the material necessary for the emission flows into a chamber that accommodates the illumination optical system and a projection optical system via the opening, adheres the carbon to an optical element in the illumination optical system and the projection optical system, and reduces the reflectance. Moreover, a light intensity on a target surface to be illuminated becomes non-uniformly according to an adhesion state of the carbon to the optical element. As a result, a decrease of throughput and resolution is caused.